The present invention relates a welding apparatus for welding a hollow stud or sleeve end-on to a workpiece surface.
DE 44 00 957 C1 and DE 295 05 613 U1 disclose welding apparatus for welding sleeves to workpieces. The known apparatus comprise a housing, a sleeve holder, a magnet coil surrounding the sleeve holder, a magnetic flux return member surrounding the coil, and a magnetic field former coupled to the flux return member for producing a magnetic field with a field component that extends radially to the sleeve within the welding gap. The radial magnetic-field component generated within the gap imparts a force to the arc to move the arc along an annular path corresponding to the annular end face of the sleeve to be welded to the workpiece.
For optimum welding results, constant welding parameters, in particular constant magnetic-field parameters, are necessary. Due to the relatively long welding and coil operating times (up to 1 s in the case of a 30 mm sleeve diameter), the magnet coil and field former are heated. This tends to affect the magnetic-field parameters, thus leading to changes in the arc movement and ultimately impairing quality of the weld. In conventional welding apparatus, therefore, sufficiently long periods are required for cooling the welding head between successive welding operations.
JP 10024368 A shows a welding torch surrounded by a magnetic coil which generates a magnetic field around the torch to agitate the molten metal. The coil is scavenged by a cooling fluid.
U.S. Pat. No. 5,508,492 A discloses an apparatus for graining the surface of a travelling electrically conductive workpiece by means of an arc which is produced by a coil embedded in a ceramic member. Cooling is provided by a coolant tube inserted in a groove that is formed in a further structural part surrounding the ceramic member.
It is an object of the invention to avoid at least some of the disadvantages as occur in comparable prior art welding apparatus. As a more specific object the invention seeks to provide may a welding apparatus which permits the welding of sleeves to workpieces with a high cycle rate and high quality.
To meet the above object, the present invention provides a welding apparatus for welding a sleeve to a workpiece. The apparatus comprises a housing, a sleeve holder for holding a sleeve so as to define a welding gap between the sleeve and the workpiece, a magnet coil surrounding the sleeve holder, a magnetic flux return member surrounding the magnet coil, a magnetic field former coupled to the magnetic flux return member for producing a magnetic field with a field component that extends radially to the sleeve within the welding gap, and cooling means for cooling the magnet coil. The cooling means includes an annular groove formed in the housing and having its inner side closed off by the magnetic flux return member, and means for supplying a cooling fluid to the annular groove.
The cooling of the magnet coil according to the invention is effective to produce a constant magnetomotive force of the field former and a more uniform magnetic field in the welding gap, resulting in an improved reproducibility and reliability of the welds and simultaneously permitting high cycle rates.
In a preferred embodiment, the magnetic flux return member and the housing are made of a heat-conducting material, the housing material being preferably aluminum. This further improves the cooling of the magnet coil and results in a structure that is easy to assemble.
The annular groove may be connected to a temperature-regulating unit so that the cooling fluid is supplied at a temperature a few degrees above the ambient temperature. Adequate cooling can thus be achieved without the danger of dew forming of in the welding region.
In accordance with another advantageous embodiment, the field former is exposed to a flow of shielding gas. The housing may have an apron-like extension surrounding the field former so as to define an intermediate space between the extension and the field former, and the shielding gas may be supplied to such intermediate space. Further, the field former may have through-openings for passing the shielding gas into its interior space. A flow of shielding gas may thus be utilized for cooling purposes.
In addition, the sleeve holder may have an axially extending through-bore for the passage of shielding gas.
Further objects, features and advantages will appear from the following description of a preferred embodiment with reference to the drawing.